The invention is directed to a laser diode with buried active layer and lateral current limitation and is also directed to a method for the manufacture thereof.
Field of the Invention
It is known to manufacture laser diodes having low threshold current and high differential efficiency as buried double hetero-structure lasers (BH) even given high output power. (See, for example, G. Winstel, C. Weyrich, Optoelektronik 1, Springer Verlag, Berlin, Heidelberg, N.Y., 1980, Pages 225 through 232). The active layer of this laser in the form of a narrow stripe is embedded in a semiconductor material having a low refractive index and a higher energy band gap. The double hetero-structure and the embedding of the active layer produce a low threshold current and a guidance of the beam in transverse and in lateral direction, so that the losses of the laser power due to diffraction in passive regions are kept low. The efficiency is increased by lateral current limitation to the narrow region of the active layer insofar as possible. A shunt current flows outside the active layer and a high output power is not achieved given incomplete lateral current limitation, for example, only due to the different threshold voltages of the hetero-pn-junction and of the homo-pn-junction. High output powers can be achieved by additional pn-junctions polarized in non-conducting direction laterally of the active stripe. The pn-junctions, however, represent additional parasitic capacitances that deteriorate the modulation behavior of the laser diode.
It is known to reduce shunt currents and parasitic capacitances in that the effective areas of the corresponding pn-junctions are reduced. For example, this occurs by etching the corresponding layers off or in that these layers are separated from the current-carrying region by etching channels. The structures resulting therefrom are not planar, this being unfavorable for manufacture and later mounting.
The publication of H. Burkhard and E. Kuphal, "Three-and Four-Layer LPE InGaAs (P) Mushroom Stripe Lasers for .lambda.=1.30, 1.54, and 1.66 um", in IEEE Journal of Quantum Electronics QE-21, 650-657 (1985) discloses a laser structure wherein channels are etched into the semiconductor material laterally of the active stripe. This structure is not a matter of a planar structure; rather, a mesa is etched onto the substrate surface and, subsequently, channels are etched out laterally of an active stripe by lateral attack of an etchant and the lower edge of this mesa. The dimensioning and lateral limitation of the active stripe is defined by the etching process. The channels are subsequently filled up with phosphorous silicate glass.
A further possibility for reducing the effective surfaces is to increase the specific resistance of the semiconductor material outside the current-carrying region, for example, by implantation of protons. The obtainable values are limited by the increase in resistance that can be achieved. One disadvantage of this method is that the resistance can again be reduced by curing of the crystal structure under the influence of heat.